1. Field of the Invention
The present invention relates to a coal thermal power plant with the aim of carbon dioxide capture and storage, and specifically relates to an oxyfuel boiler suitable for the carbon dioxide capture and storage in coal thermal power plants and a method of controlling the oxyfuel boiler.
2. Description of Related Art
As countermeasures against global warming, efforts to reduce emissions of CO2 (carbon dioxide) which is one of greenhouse gases have been actively implemented worldwide. A thermal power plant is one of such facilities that emit a large quantity of CO2, and in particular, a coal thermal power plant equipped with a coal boiler for burning coal, which contains a large amount of carbon and generates a large quantity of CO2 in the combustion flue gas, emits the largest amount of CO2 per electricity generated, and therefore, urgent CO2 reduction measures are required to be made.
In addition to the increase in efficiency in power generation, CO2 reduction measures in the coal thermal power plant include the carbon dioxide separation and capture and storage from the combustion flue gas generated by combustion in the coal boiler. The carbon dioxide separation and capture and storage is a process in which only CO2 is separated from the combustion flue gas generated in the coal thermal power plant, compressed, liquefied, and the liquefied CO2 is then sent through pipelines and stored in the deep underground.
Methods of carbon dioxide separation and capture and storage from the combustion flue gas applied to a coal thermal power plant can be roughly classified into three categories: (1) pre-combustion, (2) post-combustion, and (3) oxyfuel combustion.
The pre-combustion in method (1) is a method of carbon dioxide capture and storage from a gas mixture of H2 (hydrogen) and CO2 obtained by a reaction between coal and water steam by the physical absorption method or the like and using the remaining H2 as a fuel. Since the fuel is H2, combustion of the fuel will not generate CO2.
The post-combustion in method (2) is a method of using a membrane separation method or the like for carbon dioxide capture and storage from the combustion flue gas obtained by coal combustion in the ordinary atmospheric air (called as air combustion). In this case, major components of the combustion flue gas are N2 (nitrogen) contained in the air and CO2 generated by combustion, and those components are separated and only CO2 is captured and stored.
In contrast to the above-mentioned two carbon dioxide separation and capture and storage methods, the oxyfuel combustion in method (3) is configured such that oxygen is separated from air, and the separated pure oxygen is supplied to a coal boiler to burn fuel coal so that the coal boiler can be used as an oxyfuel boiler, then, a part of the combustion flue gas (whose major component is CO2) generated by burning coal in the oxyfuel boiler is mixed with pure oxygen, and the mixed gas is supplied to the oxyfuel boiler as oxidizing gas, thereby burning coal.
Since combustion flue gas generated in the air fired boiler contains a large amount of nitrogen (N2), it is necessary to separate carbon dioxide (CO2) from the combustion flue gas discharged from the air fired boiler. However, because most of combustion flue gas components generated in the oxyfuel boiler is CO2, the CO2 can be directly captured and stored without taking the step of separating CO2 from the flue gas.
The reason why CO2 is mixed with oxygen to form the oxidizing gas supplied to the oxyfuel boiler is to inhibit the increase in temperature of the flame burning in the coal boiler.
Because the temperature of the flame burning in the coal boiler is high during the combustion in which only oxygen is supplied to the coal boiler to burn powdered coal (pure oxygen combustion), expensive heat-resistant steel is necessary as boiler material, and the flow rate of the oxidizing gas blown out through a burner located in the coal boiler decreases, making the formation of the flame difficult. Therefore, pure oxygen combustion is not implemented in the coal boiler.
When retrofitting a coal boiler from an air combustion type to an oxygen combustion type due to the above circumstances, if the balance of heat absorption quantity in the oxygen combustion can be made equivalent to the balance of heat absorption quantity in the air combustion, retrofitting to change the area of heat transfer becomes unnecessary, which can significantly reduce the retrofit cost of the boiler.
As stated above, because the oxyfuel boiler can change the balance of heat absorption quantity in the heat exchangers and the entire heat storage quantity in the boiler by adjusting the concentration of oxygen contained in the oxidizing gas, adopting a control method which can create a condition similar to air combustion by taking advantage of that characteristic can be one idea to be considered.
For example, in a control method disclosed in Japanese Patent Laid-open No. 2007-147162, concentration of oxygen contained in the oxidizing gas is adjusted by presetting a target value of heat storage quantity in the boiler according to a load request and controlling the amount of circulating flue gas so that the actual heat storage quantity becomes the target value. Herein, the target value is set to become equivalent to the predetermined target heat storage quantity in an existing air fired boiler. Also, a preset value according to a load request is set for the oxygen supply quantity.    [Patent document 1] Japanese Patent Laid-open No. 2007-147162